A dissertation on pus

In 1785 the great English surgeon John Hunter and his Scottish colleague George Fordyce set up a medical society, the Lyceum Medicum Londinense. Its members met every fortnight in Hunter’s anatomical theatre, and the rules were fearsome: attendance was compulsory, every member present was obliged to present a paper, and there were fines for arriving late or leaving early.

Every year the Lyceum’s committee awarded a prize for the best essay submitted on a subject stipulated by the committee. In 1788 the ordained topic was one in which John Hunter had a particular interest: pus. The society’s gold medal was awarded to Hunter’s brother-in-law Everard Home, whose essay A Dissertation on the Properties of Pus was deemed so important that it subsequently appeared as a book.

What, one may ask, does the 18th-century surgeon want to learn about pus? All is revealed on page 1:

It may not…be improper to take notice, that the most generally established opinion of the nature of Pus, was, its being composed of both solids and fluids. It was distinguished by the term, ‘True, or laudable Pus’; and was supposed to differ materially from a similar discharge called, Mucus. Yet the distinctions between Pus and Mucus have been very ill defined: there was thought to be a difference, in their appearance to the eye; but the principal mark of distinction arose from a breach of surface being believed necessary to the formation of Pus, but not of Mucus; consequently, when there was no breach in the solids, the discharge was only Mucus. To ascertain the real difference between Pus and Mucus, has been considered an object meriting the attention of some of our most eminent surgeons, although they have not yet been fortunate enough to discover it.

It’s a shame that the term ‘laudable pus’ did not survive the eighteenth century. But how enlightening to discover that the pus/mucus distinction was such a lively topic at this date.

Mr Home attempts to be more specific about the subject of his disquisition:

It is difficult to give a definition of any thing, the properties of which are not well ascertained; but as it is necessary that I should particularize the substance which I propose to investigate under the term Pus, I shall define it to be a whitish fluid, made up of globules, and a transparent aqueous liquor. Pus, taken from a healthy fore, near the source of the circulation, as on the arm or breast, readily separates from the surface of the sore, the granulations underneath being small, pointed, and of a florid red colour, and has the following properties: It is nearly of the consistence of cream; is of a white colour; has a maukish taste;

Its taste? These eighteenth-century surgeons could not be accused of shrinking from an unpleasant challenge, evidently.

and when cold, is inodorous; but when warm, has a peculiar smell. Examined in the microscope, it is found to consist of two parts, of globules, and a transparent colourless fluid: the globules are, probably, white, at least they appear to have some degree of opacity: its specific gravity is greater than that of water: it does not readily go into putrefaction: exposed to heat, it evaporates to dryness; but does not coagulate: it does not unite with water in the cold of the atmosphere, but falls to the bottom; yet, if kept in a considerable degree of heat, rises, and diffuses through the water, and remains mixed with it, even after having been allowed to cool; the globules being decomposed.

Home then repeats an important observation from the work of his brother-in-law: John Hunter had previously shown that pus can be distinguished from animal mucus by the fact that the latter is flaky. But there are other important distinctions to make:

Milk is composed of globules, nearly of the same size as those of Pus; but much more numerous. Milk coagulates by runnet; which Pus does not; and contains oil and sugar, which are not to be discovered in Pus.

Helpful if you were in any danger of confusing the two.

The cases in which Pus is formed, are, properly speaking, all reducible to one, which is, the state of parts consequent to inflammation. For as far as I yet know, the fluid which I have considered as Pus, in this dissertation, has, in no instance been met with, unless preceded by inflammation; and although, in some cases, a fluid has been formed, independent of preceding inflammation, it differs from Pus in many of its properties, as has been already observed.

There follows a series of experiments, most of which Mr Home would struggle to get past a modern ethics committee. He deliberately induces an infected cyst in one healthy young man, in order to have a ready source of pus to examine under the microscope at hourly intervals. Another still braver volunteer agrees to undergo this ordeal:

A common bougie [a thin surgical instrument], four inches long, was introduced into the urethra of a healthy young man. The surface of the bougie was not oiled, which made the irritation more violent, and prevented there being any ambiguity in the appearance of the fluid collected upon it.

The bougie was removed at hourly intervals; Mr Home reports with satisfaction that after five hours ‘true pus’ was visible. The young man’s reaction to the success of the experiment is not recorded.

So what has Mr Home learned? Most significantly:

I shall briefly state the arguments which appear to have most weight, in support of Pus being a secretion from the blood. In its chemical analysis, it is found to contain similar substances with the blood. It is, in a recent state, free from any tendency to putrefaction. It is always in harmony with the parts which form it, having no power of irritating them, even when the surrounding parts are affected by it.

And he finishes with a carefully-judged acknowledgement of the research of his mentor, patron and brother-in-law – a canny move, given that Hunter was also judging this competition:

It is composed of globules swimming in a transparent fluid; which is the case with many secretions.

‘Globules swimming in a transparent fluid’ may not satisfy those of you curious to know what pus really is, so you may like to know that the fluid is known as liquor puris and rich in proteins; the globules are dead white blood cells. And it seems that – in medical research, at least – pus is making a comeback.